Summary

1. The review considers information from mammalian embryology relevant to X-chromosome inactivation, and from X-inactivation relevant to mammalian embryology.



2. Properties of the inactive-X, by which it may be recognized are: sex chromatin, heteropycnosis, late replication and the absence of gene product. Each of these has advantages and disadvantages in particular circumstances. In some species the X carries constitutive heterochromatin, which must be distinguished from the facultative region.



3. The time of X-chromosome inactivation can be estimated from the time of appearance of sex chromatin or late replication, or inferred from the appearance of heterozygotes for X-linked genes or of experimental chimaeras. The estimated time varies with species, and in the mouse and rabbit is near the time of increase in RNA synthesis.



4. Whereas in eutherian mammals either the maternally or the paternally derived X may be inactivated in different cell lines, in marsupials the paternal X is always the inactive one.



5. During development various factors act to distort the patterns produced by random X-inactivation. These factors include cell selection, transfer of gene product, and migration and mingling of cells.



6. There is no clear evidence that X-chromosome inactivation is not complete.



7. In female germ cells both X-chromosomes appear to be active. In male ones both X and Y appear inactive during most of spermatogenesis, although probably in early stages all X chromosomes present are active.



8. The active and inactive X-chromosomes may be differentiated by presence or absence of some non-histone protein or other polyanionic substance.



9. If the genes concerned in synthesis or attachment of this substance are on the X-chromosome then the differentiation will be self-maintaining.



10. The initiation of the differentiation requires either the attachment of different X-chromosomes to different sites, or some interaction of X-linked and autosomal genes, concerned in inducing or repressing activity. Some possible models are discussed.

X-chromosome inactivation and developmental patterns in mammals

I Evolution of Vertebrate Sex Chromosomes and Genomes- 1 Evidence Indicating that the X and the Y or the Z and the W were Originally an Homologous Pair of Ordinary Chromosomes- 2 Differentiation of the Primitive Sex Elements at the Expense of the Y or W, and the Conservation of the Original X or Z- 3 Polyphyletic Evolution of Vertebrates- 4 Conservation of the Original X and Homology of the X-linked Genes in Placental Mammals- 5 Conservation of the Original Z-Chromosome by Diverse Avian Species and Homology of the Z-linked Genes- II Evolution of Dosage Compensation Mechanism for Sex-linked Genes- 6 The Basic Difference in Constitution between the Mammalian X and the Drosophila X- 7 The Two Different Means of Achieving Dosage Compensation for X-linked Genes Employed by Drosophila and Mammals- 8 Various Consequences of the Dosage Compensation by X-inactivation- 9 The Conservation of the Original X and Dosage Compensation in the Face of X-polysomy- 10 Three Different Consequences of X-autosome Translocation- 11 The Consequences of Y-autosome Translocation and the XO as the Normal Female of Certain Mammalian Species- 12 Apparent Absence of Dosage Compensation for Z-linked Genes of Avian Species- III On So-called Sex Determining Factors and the Act of Sex Determination- 13 Elucidation of So-called Sex-determining Factors- 14 Time and Place of Action of Sex-determining Factors in Ontogeny- Author Index

Sex Chromosomes and Sex-Linked Genes

Mammalian X chromosome inactivation is generally considered to be a good example of stable transcriptional repression; however, there has been no satisfactory evidence for transcriptional control. We have made a test of the hypothesis of transcriptional control by Northern blot analysis of RNA from a woman heterozygous for a mutant Hpt allele which shows no detectable transcription of wild-type mRNA. Cells from this Hpt+ Hpt- woman were separated into HPRT+ and HPRT- subpopulations by selection in HAT or thioguanine. The HPRT+ population (in which the Hpt+ is on the active X) transcribed normal Hpt mRNA, while the HPRT- population (in which the Hpt+ allele is on the inactive X) did not. These results provide strong support for the hypothesis of transcriptional control.

Mammalian X chromosome inactivation: Testing the hypothesis of transcriptional control

The mode of action of sex-linked genes in mammals has been the subject of much discussion in recent years. This has centred round the concept that in the mammalian female, either the maternal or the paternal X-chromosome is randomly and irreversibly inactivated during development, with the result that in the female, as in the male, one X-chromosome only is active in a given cell (Lyon, 1966, and earlier papers). The Lyon Hypothesis (l.h.) is based, in part, on the mottled or striped phenotypes of heterozygotes for certain sex-linked genes affecting the coat of the mouse and other mammals: these phenotypes are regarded as a mixture of clones descended from cells in which either one or the other X-chromosome has been inactivated. As Baker (1967) rightly insists, the interpretation of a pattern as clonal must be based on known cell lineages, as without such a basis one simply postulates what should be put to the test.

Threshold phenomena versus cell heredity in the manifestation of sex-linked genes in mammals

Genetic and related effects of Vinca rosea alkaloids.

Studies on Rodent Chromosomes

The functional status of the X chromosome in Acheta domesticus has been analysed at the whole chromosome level on the basis of (1) 3H-thymidine autoradiography, (2) 5-BrdU/AO fluorescence microscopy, (3) in vivo 5-BrdU incorporation and (4) 3H-UdR induced aberrations. The rationale of these techniques in relation to the functional aspect of the X chromosome is that the inactive X chromosome would (1) show asynchrony in DNA synthesis, (2) show differential fluorescence, (3) respond differentially to in vivo 5-BrdU treatment and (4) the active X chromosome would show aberrations when treated with 3H-Uridine. From the results, it appears that the X chromosomes in both male (XO) and female (XX) somatic cells of Acheta are euchromatic (active). Further, the single X in the male is transcriptionally as active as the two X chromosomes in the female. In other words, the single X in the male is hyperactive when compared with the single X in the female. From this it is inferred that the male X chromosome is differentially regulated in order to bring about an equalization of it's gene product(S) to that produced by both Xs in the female. Drosophila melanogaster has a comparable system of dosage compensation. Thus, Acheta is yet another insect showing evidence for an X chromosome regulatory mechanism of dosage compensation. Additionally, it is surmised that sex determination in Acheta is based on an autosomes/X chromosome balance mechanism.

Insect sex chromosomes

The karyotype ofTatera indica cuverii consists of 68 chromosomes with two X chromosomes in the female and an X and a Y chromosome in the male. The X chromosomes are the largest and can be distinguished from each other morphologically. Autoradiographic studies indicate that the two morphologically distinguishable X chromosomes are out-of-phase in DNA replication in approximately equal proportion and thus substantiate the random inactivation hypothesis.

Studies on rodent chromosomes. II. Autoradiographic study of the sex chromosomes of the Indian gerbil, Tatera indica cuverii (Waterhouse) and its bearing on the Lyon hypothesis.

Cytological and Autoradiographic Studies of the Effects of Vincaleukoblastine (VLB) on the Root Meristem Cells of Vicia faba

Insect sex chromosomes: IX. X chromosome linkage of glucose 6-phosphate dehydrogenase (G-6-PD) locus in Gryllotalpa fossor (orthoptera)

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S. R. V. Rao

Papers

Studies on Rodent Chromosomes

Insect sex chromosomes

Studies on rodent chromosomes. II. Autoradiographic study of the sex chromosomes of the Indian gerbil, Tatera indica cuverii (Waterhouse) and its bearing on the Lyon hypothesis.

Cytological and Autoradiographic Studies of the Effects of Vincaleukoblastine (VLB) on the Root Meristem Cells of Vicia faba

Insect sex chromosomes: IX. X chromosome linkage of glucose 6-phosphate dehydrogenase (G-6-PD) locus in Gryllotalpa fossor (orthoptera)